The present invention relates generally to imaging methods and systems useful in the detection of hidden objects in turbid, i.e. scattering media and more particularly to a novel imaging method and system of the type described above having improved spatial resolution characteristics.
Recent cancer statistics (NIH publication No. 89-2789) indicate that breast cancer has the highest incidence rate (337.4/100,000) and the highest mortality rate (95.2/100,000) of any cancer in the United States since 1973. At present, early detection of breast cancer offers the best hope for improving the survival rate. X-ray mammography is the most common and currently the most reliable diagnostic tool for quantitatively diagnosing breast cancers. Despite its well-demonstrated usefulness, mammography has some important drawbacks: (1) ionizing radiation (x-rays) is utilized for imaging the breast--such radiation may be a cause of breast cancer induction, Particularly with women having an AT-gene, and/or may pose a radiation risk to young women of child bearing age; (2) images of dense of thick breasts are difficult to obtain; and (3) small growths, i.e., 1 millimeter or less, in the early stages of cancer cannot easily be detected.
Transillumination is another technique which may be used to image breast tumors. In transillumination, visible light is incident on one side of the breast and a shadow image of the tumor is recorded on the opposite side. However, the ability to observe an image thus formed is severely limited by light scattering in the breast, which reduces the intensity of the unscattered light used to form the image shadow ant contributes to noise. The reduction of the unscattered light with respect to the scattered light limits transillumination as a viable technique with which to detect breast cancer to a spatial resolution greater than 1 cm. Consequently, when a tumor is either very small (e.g. a few millimeters) or lies deep inside a breast, it may not be observed by transillumination.
To improve the detectability of small tumors located inside a breast using transillumination, one must separate the unscattered light from the scattered light. This may be done by exploiting the properties of photon migration through a scattering medium. When photons migrate through a turbid medium, there are three main signal components: (1) diffusive (incoherent) photons; (2) ballistic (coherent, forward-scattered) photons that arrive first by traveling over the shortest path; and (3) snake (quasi-coherent) photons that arrive within the first dt after traveling over relatively short paths. The diffusive scattered photons of the signal travel over a much larger distance in turbid samples than the ballistic or snake photons, which take shorter paths through the medium within a small forward angular cone. It is believed that the ballistic and snake components contain the least distorted image information and that the diffusive component loses most of the image information.
Therefore, to see through a random medium, one should selectively detect the ballistic and snake-like photons, which contain information about the object, and reject diffuse photons, which only contribute -noise. This process of selection and rejection has been done using various time-resolved techniques (see, e.g., Delpy et. al., Phys. Med. Biol., Vol. 33, 1433 (1988); Yoo et. al., Appl. Opt., Vol. 28, 2344 (1989); Hoet et. al., Appl. Opt., Vol. 28, 2304 (1989); Yoo et. al., Opt. Lett., Vol. 15, 276 (1990); Hebden et. al., Appl. Opt., Vol. 30, 788 (1991); Andersson-Engels, Opt. Lett., Vol. 15, 1179 (1990); Spears et. al. , IEEE Trans. Biomed. Eng. , Vol. 36, 1210 (1989); and Rebane et. al. , Nature Vol. 351 378 (1991) ) .
Other patents and publications of interest include U.S. Pat. No. 5,140,463 to Yoo et. al., issuing Aug. 18, 1992 (U.S. patent application Ser. No. 07/489,942 filed Mar. 8, 1990); U.S. Pat. No. 4,945,239 to Wist et. al., issued Jul. 31, 1990; Wang et. al. , Science, Vol. 253, 769 (1991); Alfano et. al., Physics World, 37 (January 1992); and Wist et. al., "A Light Imaging Technique for the Improved Detection of Breast Cancer," Eighth Southern Biomedical Engineering Conference, Richmond, Va. (Oct. 15-16, 1989) .